Which of the following physical quantities has no unit?

Options

A. Relative density
B. Density
C. Pressure
D. Thrust

Answer

To solve the question "Which of the following physical quantities has no unit?", we will analyze the options provided and determine which one does not have a unit.

### Step-by-Step Solution:

1. **Understanding Relative Density**:
- Relative density is defined as the ratio of the density of a substance to the density of a standard substance (usually water for liquids and solids, and air for gases).
- Mathematically, it can be expressed as:
\[ \text{Relative Density} = \frac{\text{Density of Substance}}{\text{Density of Standard}} \]
- Since both the numerator and denominator are densities, they have the same units (e.g., kg/m³ or g/cm³). Therefore, when you divide them, the units cancel out.

2. **Analyzing Density**:
- Density is defined as mass per unit volume:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} = \frac{\text{kg}}{\text{m}^3} \]
- Density has units (e.g., kg/m³).

3. **Analyzing Pressure**:
- Pressure is defined as force per unit area:
\[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} = \frac{\text{Newton}}{\text{m}^2} \]
- Pressure has units (e.g., N/m² or Pascal).

4. **Analyzing Thrust**:
- Thrust is a force and can be calculated as pressure multiplied by area:
\[ \text{Thrust} = \text{Pressure} \times \text{Area} \]
- Since pressure has units (N/m²) and area has units (m²), thrust also has units (N).

5. **Conclusion**:
- Among all the options analyzed, only relative density has no units because it is a ratio of two densities that cancel each other out.
- Therefore, the correct answer is **Relative Density**.

### Final Answer:
The physical quantity that has no unit is **Relative Density**.